Popular Science Monthly/Volume 41/May 1892/A Desert Fruit
|A DESERT FRUIT.|
By GRANT ALLEN.
WHO knows the Mediterranean, knows the prickly pear. Not that that quaint and uncanny-looking cactus, with its yellow blossoms and bristling fruits that seem to grow paradoxically out of the edge of thick, fleshy leaves is really a native of Italy, Spain, and North Africa, where it now abounds on every sun-smitten hillside. Like Mr. Henry James and Mr. Marion Crawford, the Barbary fig, as the French call it, is, in point of fact, an American citizen, domiciled and half naturalized on this side of the Atlantic, but redolent still at heart of its Columbian origin. Nothing is more common, indeed, than to see classical pictures of the Alma-Tadema school — not, of course, from the brush of the master himself, who is impeccable in such details, but fair works of decent imitators — in which Caia or Marcia leans gracefully in her white stole on one pensive elbow against a marble lintel, beside a court-yard decorated with a Pompeiian basin, and overgrown with prickly pear or "American aloes." I need hardly say that, as a matter of plain historical fact, neither cactuses nor agaves were known in Europe till long after Christopher Columbus had steered his wandering bark to the sandy shores of Cat's Island in the Bahamas. (I have seen Cat's Island with these very eyes, and can honestly assure you that its shores are sandy.) But this is only one among the many pardonable little inaccuracies of painters, who thrust scarlet geraniums from the Cape of Good Hope into the fingers of Aspasia, or supply King Solomon in all his glory with Japanese lilies of the most recent introduction.
At the present day, it is true, both the prickly-pear cactus and the American agave (which the world at large insists upon confounding with the aloe, a member of a totally distinct family) have spread themselves in an apparently wild condition over all the rocky coasts both of southern Europe and of northern Africa. The alien desert weeds have fixed their roots firmly in the sunbaked clefts of Ligurian Apennines; the tall candelabrum of the Western agave has reared its great spike of branching blossoms (which flower, not once in a century, as legend avers, but once in some fifteen years or so) on all the basking hillsides of the Mauritanian Atlas. But for the origin, and therefore for the evolutionary history, of either plant, we must look away from the shore of the inland sea to the arid expanse of the Mexican desert. It was there, among the sweltering rocks of the Tierras Calientes, that these ungainly cactuses first learned to clothe themselves in prickly mail, to store in their loose tissues an abundant supply of sticky moisture, and to set at defiance the persistent attacks of all external enemies. The prickly pear, in fact, is a typical instance of a desert plant, as the camel is a typical instance of a desert animal. Each lays itself out to endure the long droughts of its almost rainless habitat by drinking as much as it can when opportunity offers, hoarding up the superfluous water for future use, and economizing evaporation by every means in its power.
If you ask that convenient fiction, the Man in the Street, what sort of plant a cactus is, he will probably tell you it is all leaf and no stem, and each of the leaves grows out of the last one. Whenever we set up the Man in the Street, however, you must have noticed we do it in order to knock him down again like a nine-pin next moment: and this particular instance is no exception to the rule; for the truth is that a cactus is practically all stem and no leaves, what looks like a leaf being really a branch sticking out at an angle. The true leaves, if there are any, are reduced to mere spines or prickles on the surface, while the branches, in the prickly-pear and many of the ornamental hot-house cactuses, are flattened out like a leaf to perform foliar functions. In most plants, to put it simply, the leaves are the mouths and stomachs of the organism; their thin and flattened blades are spread out horizontally in a wide expanse, covered with tiny throats and lips which suck in carbonic acid from the surrounding air, and disintegrate it in their own cells under the influence of sunlight. In the prickly pears, on the contrary, it is the flattened stem and branches which undertake this essential operation in the life of the plant the sucking-in of carbon and giving-out of oxygen, which is to the vegetable exactly what the eating and digesting of food is to the animal organism. In their old age, however, the stems of the prickly pear display their true character by becoming woody in texture and losing their articulated leaf-like appearance.
Everything on this earth can best be understood by investigating the history of its origin and development, and in order to understand this curious reversal of the ordinary rule in the cactus tribe we must look at the circumstances under which the race was evolved in the howling waste of American deserts. (All deserts have a prescriptive right to howl, and I wouldn't for worlds deprive them of the privilege.) Some familiar analogies will help us to see the utility of this arrangement. Everybody knows our common English stone-crops or if he doesn't he ought to, for they are pretty and ubiquitous. Now, stone-crops grow for the most part in chinks of the rock or thirsty, sandy soil; they are essentially plants of very dry positions. Hence they have thick and succulent little stems and leaves, which merge into one another by imperceptible gradations. All parts of the plant alike are stumpy, green, and cylindrical. If you squash them with your finger and thumb you find that, though the outer skin or epidermis is thick and firm, the, inside is sticky, moist, and jelly-like. The reason for all this is plain: the stone-crops drink greedily by their roots whenever they get a chance, and store up the water so obtained to keep them from withering under the hot and pitiless sun that beats down upon them for hours in the baked clefts of their granite matrix. It's the camel trick over again. So leaves and stems grow thick and round and juicy within; but outside they are inclosed in a stout layer of epidermis, which consists of empty glassy cells, and which can be peeled off or flayed with a knife like the skin of an animal. This outer layer prevents evaporation, and is a marked feature of all succulent plants which grow exposed to the sun on arid rocks or in sandy deserts.
The tendency to produce rounded stems and leaves, little distinguishable from one another, is equally noticeable in many seaside plants which frequent the strip of thirsty sand beyond the reach of the tides. That belt of dry beach that stretches between high-water mark and the zone of vegetable mold is to all intents and purposes a miniature desert. True, it is watered by rain from time to time; but the drops sink in so fast that in half an hour, as we know, the entire strip is as dry as Sahara again. Now, there are many shore weeds of this intermediate sand-belt which mimic to a surprising degree the chief external features of the cactuses. One such weed, the common salicornia, which grows in sandy bottoms or hollows of the beach, has a jointed stem, branched and succulent, after the true cactus pattern, and entirely without leaves or their equivalents in any way. Still more cactus-like in general effect is another familiar English seaside weed, the kali or glasswort, so called because it was formerly burned to extract the soda. The glasswort has leaves, it is true, but they are thick and fleshy, continuous with the stem, and each one terminating in a sharp, needle-like spine, which effectually protects the weed against all browsing aggressors.
Now, wherever you get very dry and sandy conditions of soil, you get this same type of cactus-like vegetation—plantes grasses, as the French well call them. The species which exhibit it are not necessarily related to one another in any way; often they belong to most widely distinct families; it is an adaptive resemblance alone, due to similarity of external circumstances only. The plants have to fight against the same difficulties, and they adopt for the most part the same tactics to fight them with. In other words, any plant, of whatever family, which wishes to thrive in desert conditions, must almost as a matter of course become thick and succulent, so as to store up water, and must be protected by a stout epidermis to prevent its evaporation under the fierce heat of the sunlight. They do not necessarily lose their leaves in the process; but the jointed stem usually answers the purpose of leaves under such conditions far better than any thin and exposed blade could do in the arid air of a baking desert. And therefore, as a rule, desert plants are leafless.
In India, for example, there are no cactuses. But I wouldn't advise you to dispute the point with a peppery, fire-eating AngloIndian colonel. I did so once, myself, at the risk of my life, at a table d'hôte on the Continent; and the wonder is that I'm still alive to tell the story. I had nothing but facts on my side, while the colonel had fists, and probably pistols. And when I say no cactuses, I mean, of course, no indigenous species; for prickly pears and epiphyllums may naturally be planted by the hand of man anywhere. But what people take for thickets of cactus in the Indian jungle are really thickets of cactus-like spurges. In the dry soil of India, many spurges grow thick and succulent, learn to suppress their leaves, and assume the bizarre forms and quaint jointed appearance of the true cactuses. In flower and fruit, however, they are euphorbias to the end; it is only in the thick and fleshy stem that they resemble their nobler and more beautiful Western rivals. No true cactus grows truly wild anywhere on earth except in America. The family was developed there, and, till man transplanted it, never succeeded in gaining a foothold elsewhere. Essentially tropical in type, it was provided with no means of dispersing its seeds across the enormous expanse of intervening ocean which separated its habitat from the sister continents.
But why are cactuses so almost universally prickly? From the grotesque little melon-cactuses of our English hot-houses to the huge and ungainly monsters which form miles of hedgerows on Jamaican hillsides, the members of this desert family are mostly distinguished by their abundant spines and thorns, or by the irritating hairs which break off in your skin if you happen to brush incautiously against them. Cactuses are the hedgehogs of the vegetable world; their motto is Nemo me impune lacessit. Many a time in the West Indies I have pushed my hand for a second into a bit of tangled "bush," as the negroes call it, to seize some rare flower or some beautiful insect, and been punished for twenty-four hours afterward by the stings of the almost invisible and glass-like little cactus-needles. When you rub them they only break in pieces, and every piece inflicts a fresh wound on the flesh where it rankles. Some of the species have large, stout prickles; some have clusters of irritating hairs at measured distances; and some rejoice in both means of defense at once, scattered impartially over their entire surface. In the prickly pear, the bundles of prickles are arranged geometrically with great regularity in a perfect quincunx. But that is a small consolation indeed to the reflective mind when you've stung yourself badly with them.
The reason for this bellicose disposition on the part of the cactuses is a tolerably easy one to guess. Fodder is rare in the desert. The starving herbivores that find themselves from time to time belated on the confines of such thirsty regions would seize with avidity upon any succulent plant which offered them food and drink at once in their last extremity. Fancy the joy with which a lost caravan, dying of hunger and thirst in the byways of Sahara, would hail a great bed of melons, cucumbers, and lettuces! Needless to say, however, under such circumstances melon, cucumber, and lettuce would soon be exterminated; they would be promptly eaten up at discretion without leaving a descendant to represent them in the second generation. In the ceaseless war between herbivore and plant, which is waged every day and all day long the whole world over with far greater persistence than the war between carnivore and prey, only those species of plant can survive in such exposed situations which happen to develop spines, thorns, or prickles as a means of defense against the mouths of hungry and desperate assailants.
Nor is this so difficult a bit of evolution as it looks at first sight. Almost all plants are more or less covered with hairs, and it needs but a slight thickening at the base, a slight woody deposit at the point, to turn them forthwith into the stout prickles of the rose or the bramble. Most leaves are more or less pointed at the end or at the summits of the lobes; and it needs but a slight intensification of this pointed tendency to produce forthwith the sharp defensive foliage of gorse, thistles, and holly. Often one can see all the intermediate stages still surviving under one's very eyes. The thistles themselves, for example, vary from soft and unarmed species which haunt out-of-the-way spots beyond the reach of browsing herbivores, to such trebly-mailed types as that enemy of the agricultural interest, the creeping thistle, in which the leaves continue themselves as prickly wings down every side of the stem, so that the whole plant is amply clad from head to foot in a defensive coat of fierce and bristling spear-heads. There is a common little English meadow weed, the rest-harrow, which in rich and uncropped fields produces no defensive armor of any sort; but on the much-browsed-over suburban commons and in similar exposed spots, where only gorse and blackthorn stand a chance for their lives against the cows and donkeys, it has developed a protected variety in which some of the branches grow abortive, and end abruptly in stout spines like a hawthorn's. Only those rest-harrows have there survived in the sharp struggle for existence which happened most to baffle their relentless pursuers.
Desert plants naturally carry this tendency to its highest point of development. Nowhere else is the struggle for life so fierce; nowhere else is the enemy so goaded by hunger and thirst to desperate measures. It is a place for internecine warfare. Hence, all desert plants are quite absurdly prickly. The starving herbivores will attack and devour under such circumstances even thorny weeds, which tear or sting their tender tongues and palates, but which supply them at least with a little food and moisture: so the plants are compelled in turn to take almost extravagant precautions. Sometimes the leaves end in a stout dagger-like point, as with the agave, or so-called American aloe; sometimes they are reduced to mere prickles or bundles of needle-like spikes; sometimes they are suppressed altogether, and the work of defense is undertaken in their stead by irritating hairs intermixed with caltrops of spines pointing outward from a common center in every direction. When one remembers how delicately sensitive are the tender noses of most browsing herbivores, one can realize what an excellent mode of defense these irritating hairs must naturally constitute. I have seen cows in Jamaica almost maddened by their stings, and even savage bulls will think twice in their rage before they attempt to make their way through the serried spears of a dense cactus hedge. To put it briefly, plants have survived under very arid or sandy conditions precisely in proportion as they displayed this tendency toward the production of thorns, spines, bristles, and prickles.
It is a marked characteristic of the cactus tribe to be very tenacious of life, and, when hacked to pieces, to spring afresh in full vigor from every scrap or fragment. True vegetable hydras, when you cut down one, ten spring in its place; every separate morsel of the thick and succulent stem has the power of growing anew into a separate cactus. Surprising as this peculiarity seems at first sight, it is only a special desert modification of a faculty possessed in a less degree by almost all plants and by many animals. If you cut off the end of a rose branch and stick it in the ground under suitable conditions, it grows into a rose tree. If you take cuttings of scarlet geraniums or common verbenas, and pot them in moist soil, they bud out apace into new plants like their parents. Certain special types can even be propagated from fragments of the leaf; for example, there is a particularly vivacious begonia off which you may snap a corner of one blade, and hang it up by a string from a peg or the ceiling, when, hi presto! little begonia plants begin to bud out incontinently on every side from its edges. A certain German professor went even further than that: he chopped up a liverwort very fine into vegetable mincemeat, which he then spread thin over a saucerful of moist sand, and lo! in a few days the whole surface of the mess was covered with a perfect forest of sprouting little liverworts. Roughly speaking, one may say that every fragment of every organism has in it the power to rebuild in its entirety another organism like the one of which it once formed a component element.
Similarly with animals. Cut off a lizard's tail, and straightway a new tail grows in its place with surprising promptitude. Cut off a lobster's claw, and in a very few weeks that lobster is walking about airily on his native rocks, with two claws as usual. True, in these cases the tail and the claw don't bud out in turn into a new lizard or a new lobster. But that is a penalty the higher organisms have to pay for their extreme complexity. They have lost that plasticity, that freedom of growth, which characterizes the simpler and more primitive forms of life; in their case the power of producing fresh organisms entire from a single fragment, once diffused equally over the whole body, is now confined to certain specialized cells which, in their developed form, we know as seeds or eggs. Yet, even among animals, at a low stage of development, this original power of reproducing the whole from a single part remains inherent in the organism; for you may chop up a fresh-water hydra into a hundred little bits, and every bit will be capable of growing afresh into a complete hydra.
Now, desert plants would naturally retain this primitive tendency in a very high degree; for they are specially organized to resist drought being the survivors of generations of droughtproof ancestors and, like the camel, they have often to struggle on through long periods of time without a drop of water. Exactly the same thing happens at home to many of our pretty little European stone-crops. I have a rockery near my house overgrown with the little white sedum of our gardens. The birds often pick off a tiny leaf or branch; it drops on the dry soil, and remains there for days without giving a sign of life. But its thick epidermis effectually saves it from withering; and as soon as rain falls, wee white rootlets sprout out from the under side of the fragment as it lies, and it grows before long into a fresh small sedum plant. Thus, what seem like destructive agencies themselves, are turned in the end by mere tenacity of life into a secondary means of propagation.
That is why the prickly pear is so common in all countries where the climate suits it, and where it has once managed to gain a foothold, The more you cut it down, the thicker it springs; each murdered bit becomes the parent in due time of a numerous offspring. Man, however, with his usual ingenuity, has managed to best the plant, on this its own ground, and turn it into a useful fodder for his beasts of burden. The prickly pear is planted abundantly on bare rocks in Algeria, where nothing else would grow, and is cut down when adult, divested of its thorns by a rough process of hacking, and used as food for camels and cattle. It thus provides fresh moist fodder in the African summer when the grass is dried up and all other pasture crops have failed entirely.
The flowers of the prickly pear, as of many other cactuses, grow apparently on the edge of the leaves, which alone might give the observant mind a hint as to the true nature of those thick and flattened expansions. For, whenever what look like leaves bear flowers or fruit on their edge or midrib, as in the familiar instance of butcher's broom, you may be sure at a glance they are really branches in disguise masquerading as foliage. The blossoms in the prickly pear are large, handsome, and yellow; at least, they would be handsome if one could ever see them, but they're generally covered so thick in dust that it's difficult properly to appreciate their beauty. They have a great many petals in numerous rows, and a great many stamens in a rosette in the center; and to the best of my knowledge and belief, as lawyers put it, they are fertilized for the most part by tropical butterflies; but on this point, having observed them but little in their native habitats, I speak under correction.
The fruit itself, to which the plant owes its popular name, is botanically a berry, though a very big one, and it exhibits in a highly specialized degree the general tactics of all its family. As far as their leaf-like stems go, the main object in life of the cactuses is—not to get eaten. But when it comes to the fruit, this object in life is exactly reversed; the plant desires its fruit to be devoured by some friendly bird or adapted animal, in order that the hard little seeds buried in the pulp within may be dispersed for germination under suitable conditions. At the same time, true to its central idea, it covers even the pear itself with deterrent and prickly hairs, meant to act as a defense against useless thieves or petty depredators, who would eat the soft pulp on the plant as it stands (much as wasps do peaches) without benefiting the species in return by dispersing its seedlings. This practice is fully in accordance with the general habit of tropical or subtropical fruits, which lay themselves out to deserve the kind offices of monkeys, parrots, toucans, hornbills, and other such large and powerful fruit-feeders. Fruits which arrange themselves for a clientèle of this character have usually thick or nauseous rinds, prickly husks, or other deterrent integuments; but they are full within of juicy pulp, imbedding stony or nutlike seeds, which pass undigested through the gizzards of their swallowers.
For a similar reason, the actual prickly pears themselves are attractively colored. I need hardly point out, I suppose, at the present time of day, that such tints in the vegetable world act like the gaudy posters of our London advertisers. Fruits and flowers which desire to attract the attention of beasts, birds, or insects, are tricked out in flaunting hues of crimson, purple, blue, and yellow; fruits and flowers which could only be injured by the notice of animals are small and green, or dingy and inconspicuous. — Longman's Magazine.